1. Field of the Invention
The present invention generally relates to scanning optical devices and image forming apparatuses using the scanning optical devices. More particularly, the present invention is applicable, for example, to apparatuses using electrophotographic processes, such as laser beam printers, digital multifunction machines, and facsimile machines, in which a plurality of laser beams (luminous fluxes) emitted from a plurality of light sources are reflected and deflected by optical deflectors and target surfaces are optically scanned with scanning optical lens systems so that image data is recorded at high speed.
2. Description of the Related Art
Scanning optical devices using lasers as light sources in digital multifunction machines and laser beam printers (LBPs) are commonly known. In recent years, higher-speed apparatuses have been required. Thus, higher-speed scanning optical devices have been anticipated. In order to realize higher-speed scanning optical devices, for example, the rotation speed of optical deflectors may be increased, thereby increasing the scanning speed.
For higher-speed scanning optical devices, expensive motors must be used because of the rapid rotation of the optical deflector and measures must be taken against excessive heat generation. Moreover, the light sources need to have increased light intensity. These requirements cause the component costs to increase and further increase other costs due to the complicated structure.
Thus, in recent years, lasers having a plurality of light-emitting points have been used as light sources. Also, a plurality of lasers is aligned so that parallel scanning in the sub-scanning direction is performed with a plurality of laser beams. Accordingly, higher-speed performance can be achieved while suppressing the rotation speed of the optical deflector.
In a scanning optical device using a plurality of light sources, for example, the light sources are placed on the same substrate and are separated from each other by a predetermined distance in the main scanning direction and in the sub-scanning direction. A plurality of laser beams emitted from the plurality of light sources is combined to produce a plurality of adjacent beams through an optical element such as a prism, and scanning is then performed with the plurality of adjacent beams.
FIG. 6 is a sectional view, taken along the main scanning direction, of a principal portion of a known scanning optical device using a prism for combining beams.
Referring to FIG. 6, components of two laser beams A and B that are light-modulated and emitted from two light sources 50a and 50b are converted into substantially parallel luminous fluxes (or divergent luminous fluxes) through collimator lenses 51a and 51b, respectively. The converted laser beams A and B pass through apertures 60a and 60b, respectively, and are combined together through a prism 52 to produce a plurality of adjacent beams in the main scanning cross-section. Then, the combined laser beams A and B enter a common cylindrical lens 53 having predetermined refractive power only in the sub-scanning direction, and are focused as linear images which are long in the main scanning direction on a deflection surface (reflection surface) 54a of an optical deflector 54.
The laser beams A and B reflected and deflected at the deflection surface 54a of the optical deflector 54 are focused to spots on a photosensitive drum surface 56 through a scanning optical lens 55 having an fθ characteristic. The optical deflector 54 rotates in the direction indicated by arrow C in FIG. 6, and thus the photosensitive drum surface 56 is scanned with the laser beams A and B in the direction indicated by arrow D in FIG. 6 (main scanning direction) at a constant speed. Accordingly, images are recorded on the photosensitive drum surface 56 functioning as a recording medium.
A BD detector 57 includes a BD lens 58 for detecting synchronization and a BD sensor 59. In order to adjust the timing of the starting point for scanning over the photosensitive drum surface 56 prior to optical scanning, part of the laser beams (BD luminous flux) reflected and deflected by the optical deflector 54 is guided to the BD sensor 59 through the BD lens 58. A Synchronization signal (a BD signal) obtained from a detected output signal from the BD sensor 59 allows adjustment of the timing of the starting point for scanning to record an image onto the photosensitive drum surface 56.
Referring to FIG. 6, the laser beams A and B emitted from the light sources 50a and 50b are focused at predetermined intervals in the sub-scanning direction to form a plurality of scanning lines at the same time. Thus, scanning can be performed at a higher speed as compared to a case in which scanning is performed with a single laser beam, the speed being increased by the number of scanning lines.
A scanning optical device not requiring a prism for combining beams is proposed in Japanese Patent Laid-Open No. 11-352426. In the scanning optical device of this type, principal rays of a plurality of laser beams with a predetermined angle between them in the main scanning direction, which are emitted from a plurality of light sources, enter a deflection surface of an optical deflector so as to almost meet each other on the deflection surface. Hereinafter, this arrangement is referred to as “radial arrangement”.
In the scanning optical device disclosed in Japanese Patent Laid-Open No. 11-352426, two incident beams enter the deflection surface from outside of scanning beams that are scanned by the deflection surface. Also, an under-filled scanner (UFS) optical system, in which incident luminous flux having a smaller width than the width of a deflection surface is deflected and scanned, is used. Causing the two beams to cross after the deflection surface allows a reduced amount of vignetting of each of the incident beams on the reflection surface and an increased common scanning range.
Referring to FIG. 7, in a radially arranged optical system, due to the phase difference of the angle Δθ, scanning with the laser beam B is initiated Δθ after the scanning with the laser beam A on the target surface.
If the image forming range in the main scanning direction of the laser beams A and B is large, the preceding scanning with the laser beam A by the deflection surface 54a may proceed to scanning with the next deflection surface before the scanning with the laser beam B reaches the end of the scanning by the deflection surface 54a. 
Thus, the same position is scanned with the laser beam A twice before the photosensitive drum rotates for the next sub scanning position. This causes an image to be re-recorded on the image recorded immediately before, thus deteriorating the image quality.
Although arranging a plurality of light sources with an angle Δθ between them is advantageous for realizing higher-speed performance, the relationship between the angle Δθ and the image forming range must be appropriately determined. There is a first problem in that inappropriate setting of the relationship between the angle Δθ and the image forming range causes no assurance of high quality images even if higher speed performance is achieved. In the scanning optical device disclosed in Japanese Patent Laid-Open No. 11-352426, incident beams enter a deflection surface from outside of scanning beams. Thus, even if the incident beams cross each other after the deflection surface, the increase in the common scanning range is limited.
There is a second problem in that the scanning efficiency cannot be increased. This phenomena particular to the UFS optical system is based on the following principle. Even if incident beams cross each other after a deflection surface, the common scanning range is limited. In the UFS optical system, light beams enter a deflection surface as luminous fluxes having a width in the main scanning direction smaller than that of the deflection surface. Thus, rotation of an optical deflector for deflection scanning causes an edge between deflection surfaces to affect the incident luminous flux, thus causing vignetting of the incident luminous flux. The vignetting of the luminous flux causes the diameter of the main scanning spot to drastically increase and a significantly reduced light intensity, resulting in deteriorated image quality. Thus, the upper limit of the scanning angle, that is, the scanning efficiency ρ in the UFS optical system is determined from the vignetting. Normally, the scanning efficiency ρ in the UFS optical system is at most approximately 0.7 and does not reach 0.8.
For the least vignetting of the incident luminous flux, the incident luminous flux enters from the center of the scanning range (for example, an optical axis of an fθ lens) so that the incident luminous flux enters a deflection surface as perpendicularly as possible. Accordingly, the width of the incident luminous flux projected onto a deflection surface is minimized and an arrangement least susceptible to vignetting can be achieved.
In a radially arranged optical system, using incident beams that cross after a deflection surface further increases the scanning efficiency.
In the scanning optical device disclosed in Japanese Patent Laid-Open No. 11-352426, incident beams enter a deflection surface with a large angle from outside of the scanning range, thus not providing sufficient scanning efficiency in the UFS optical system.
Moreover, the UFS optical system used, for example, in the scanning optical device disclosed in Japanese Patent Laid-Open No. 11-352426 has a third problem in that the scanning efficiency ρ cannot be set to 0.8 or more.